The Glasgow Subway (sometimes known as the Underground) is one of the world's oldest metros.

A rapid transit, underground, subway, elevated, or metro system is a railway system, usually in an urban area, that usually has high capacity and frequency, with large trains and total or near total grade separation from other traffic.

Definitions and Nomenclature

There is no single term in English that all speakers would use for all rapid transit or metro systems.
This fact reflects variations not only in national and regional usage, but in what characteristics are considered essential.

One definition of a "true" metro system is as follows:

an urban, electric mass transit railway system

totally independent from other traffic

with high service frequency.

But those who prefer the American term "subway" or the British "underground" would additionally specify that the tracks and stations must be

located below street level

so that pedestrians and road users see the street exactly as it would be without the subway; or at least that this must be true for the most important, central parts of the system.
Conversely, those who prefer the American "rapid transit" or the newer term "metro" tend to view
this as a less important characteristic and are pleased to include systems that are entirely
elevated or at ground level ("at grade") as long as the other criteria are met.
A rapid transit system that is generally above street level may be called an "elevated" system (often shortened to el or, in Chicago, 'L').
In some cities the word "subway" applies to the entire system, in others only to those parts that actually are underground; and analogously for "el".

For a more comprehensive listing of other names of this kind of system in cities around
the world, see the list of rapid transit systems. Germanic languages generally use names
meaning "underground railway", while many others use "metro".

Extent

Farringdon station in London is shared by Underground trains (foreground) and Thameslink suburban rail services (behind partition).

In larger metropolitan areas the metro system may extend only to the limits of the central city, or to its inner ring of suburbs, with trains making relatively frequent station stops. The outer suburbs may then be reached by a separate commuter, suburban, or regional rail network, where more widely spaced stations allow a higher speed. These trains are often more expensive and less frequent, sometimes operating only in rush hours, and sometimes for political reasons they are operated by a separate authority that tends not to cooperate with the city's transit authority.

Many of these regional railways were first built to operate in one direction from a city-center terminus, but some have been extended across the city center, sometimes running in tunnels. By making multiple stops in the city, they can offer suburban passengers a choice of stations, and also provide useful transportation within the city. A notable example is the ParisRER system, where (in cooperation with the city's transit authority) several pairs of existing suburban lines running in opposite directions from the city have been extended in tunnel to join up and form new through routes across the city. They are provided with frequent service and, within the city, the same fares as the Métro are charged, providing an integrated network. In German-speaking countries, such a system is called an S-Bahn.

In some cases, such as the San FranciscoBay Area Rapid Transit (BART) and WashingtonMetrorail systems, the rapid transit system itself runs to the suburbs and effectively functions as a regional rail service as well. Where there are separate systems, the rapid transit system is typically a self-contained service with its own dedicated tracks and stations and technologically incompatible with other railways. Suburban rail services, on the other hand, often share tracks and stations with long-distance trains (historically they were usually operated by the same company, which also owned the rails and ran freight, although this has become less common) and are subject to the same standards and regulations. There are exceptions; some London Underground lines share track with suburban rail services. In some cases, metro lines have been extended by taking over existing regional rail lines, notably the Central and Northern Lines in London. The Athens Metro's Blue Line shares tracks with suburban rail services in order to connect the metro to Eleftherios Venizelos International Airport, but does not stop at the suburban rail stations because the platforms for the stations are higher than the train's floor.

Most underground systems are for public transportation, but a few cities have built freight or postal lines. One example was the Post Office Railway, which transported mail underground between sorting offices in London from 1927 until it was abandoned in 2003. Similarly, in its early days the London Underground's Metropolitan Line (then the Metropolitan Railway) transported goods as well as running passenger trains. Another example was the Chicago Freight Subway, which had a dense grid of tunnels under downtown Chicago.

During the Cold War an important secondary function of some underground systems was to provide shelter in case of a nuclear attack.

Urban rail systems have often been used to showcase economical, social, and technological achievements of a nation, especially in the Soviet Union and other socialist countries. With their marble walls, polished granite floors and splendid mosaics, the metro systems of Moscow and St. Petersburg are widely regarded as some of the most beautiful in the world. Modern metro stations in Russia are usually still built with the same emphasis on appearance. Similarly, the Independent Subway System in New York City was built to compete with the private IRT and BMT systems, and succeeded in running them out of business (in conjunction with other factors, such as fare limits).

Metro stations, more so than railway and bus stations, often have a characteristic artistic design that can identify each stop. Some have sculptures or frescos. For example, London's Baker Street station is adorned with tiles depicting Sherlock Holmes. Every metro station in Valencia, Spain has a different sculpture on the ticket-hall level. Alameda station is decorated with fragments of while tile, like the dominant style of the Ciutat de les Arts i les Ciències.

Technology

This station of the Paris Métro has automatic platform doors to prevent passengers from falling onto the tracks.

Trains

Some urban rail lines are built to the full size of main-line railways; others use smaller tunnels, restricting the size and sometimes the shape of the trains (in the London Underground the informal term tube train is commonly used). Some lines use light rail rolling stock, perhaps surface cars simply routed into a tunnel for all or part of their route. In many cities, such as London and Boston's MBTA, lines using different types of vehicles are organised into a single unified system (though often not connected by track).

Although the initial lines of what became the London Underground used steam engines, most metro trains, both now and historically, are electricmultiple units, with steelwheels running on two steel rails. Power is commonly supplied by means of a single live third rail (as in New York) at 600 to 750 volts, but some systems use two live rails (notably London) and thus eliminate the return current from the running rails. Overhead wires, allowing higher voltages, are more likely to be used on metro systems without much length in tunnel, as in Amsterdam; but they also occur on some that are underground, as in Madrid.

Older systems generally use DC power rather than AC, even though this requires large rectifiers for the power supply. DC motors were formerly more efficient for railway applications, and once a DC system is in place, converting it to AC is generally considered too large a project to contemplate.

The rubber-tyred Montréal Métro is known for its quiet trains and elegant stations.

An alternative technology using rubber tyres on narrow concrete or steel rollways was pioneered on the Paris Métro, and the first complete system to use it was in Montréal. Additional horizontal wheels are required for guidance, and a conventional track is often provided in case of flat tires and for switching. Advocates of this system note that it is much quieter than conventional steel-wheeled trains, and allows for greater inclines given the increased traction allowed by the rubber tyres.

For elevated lines, still another alternative is the monorail. Supported or "straddle" monorails, with a single rail below the train, include the Tokyo Monorail; the Schwebebahn in Wuppertal is a suspended monorail, where the train body hangs below the wheels and rail. Monorails have never gained wide acceptance outside Japan, though Seattle has a short one, which it hopes to replace with a new, larger system, and one has recently been built in Las Vegas.

Drivers and automation

Early underground trains often carried an attendant on each car to operate the doors or gates, as well as a driver. The introduction of powered doors around 1920 permitted crew sizes to be reduced, and trains in many cities are now operated by a single person. Where the operator would not be able to see the whole side of the train to tell whether the doors can be safely closed, mirrors or closed-circuit TV monitors are often provided for that purpose.

An alternative to human drivers became available in the 1960s, as automated systems were developed that could start a train, accelerate to the correct speed, and stop automatically at the next station, also taking into account the information that a human driver would obtain from lineside or cab signals.
The first complete line to use this technology was London's Victoria Line, in 1968.
In normal operation the one crew member sits in the driver's position at the front, but just closes the doors at each station; the train then starts automatically. This style of system has become widespread. A variant is seen on London's Docklands Light Railway,
opened in 1987, where the "passenger service agent" (formerly "train captain") rides with the passengers rather than sitting at the front as a driver would.

The same technology would have allowed trains to operate completely automatically with no crew, just as most elevators do; and as the cost of automation has decreased, this has become financially attractive.
But a countervailing argument is that of possible emergency situations. A crew member on board the train may be able to prevent the emergency in the first place, drive a partially failed train to the next station, assist with an evacuation if needed, or call for the correct emergency services (police, fire, or ambulance) and help direct them.
In some cities the same reasons are considered to justify a crew of two rather than one; one person drives from the front of the train, while the other operates the doors from a position farther back, and is more conveniently able to assist passengers in the rear cars. The crew members may exchange roles on the reverse trip (as in Toronto) or not (as in New York).

Completely unmanned trains are more accepted on newer systems where there are no existing crews to be removed, and especially on light rail lines.
Thus the first such system was the VAL (véhicule automatique léger or "automated light vehicle") of Lille, France, inaugurated in 1983.
Additional VAL lines have been built in other cities.
In Canada, the VancouverSkyTrain carries no crew members, while Toronto'sScarborough RT, opening the same year (1985) with otherwise identical trains, uses human operators.

In stations for unmanned trains, the entire platform is sometimes screened from the track by a wall, typically of glass,
with automatic platform-edge doors (PEDs) that open, like elevator doors, when a train is stopped. This eliminates the hazard that a passenger will accidentally fall
(or deliberately jump) onto the tracks and be run over or electrocuted, and it also gives better control over ventilation
of the platform. But PEDs are not always used even on unmanned lines: they add cost and complexity to the system, and trains may have to approach the station more
slowly so they can stop in accurate alignment with the doors.
Conversely, some stations where trains do have drivers are provided with PEDs because of their other advantages:
for example, part of London's Jubilee Line.

Tunnel construction

The construction of an underground metro is an expensive project, often carried out over a number of years. There are several different methods of building underground lines.

In one common method, known as cut-and-cover, the city streets are excavated and a tunnel structure strong enough to support the road above is built at the trench, which is then filled in and the roadway rebuilt. This method (used for most of the underground parts of the São Paulo metro, for example) often involves extensive relocation of the utilities commonly buried not far below city streets – particularly power and telephone wiring, water and gas mains, and sewers. The structures are typically made of concrete, perhaps with structural columns of steel; in the oldest systems, brick and cast iron were used. Cut-and-cover construction can take so long that it is often necessary to build a temporary roadbed while construction is going on underneath in order to avoid closing main streets for long periods of time; in Toronto, a temporary surface on Yonge Street supported cars and streetcar tracks for several years while the Yonge subway was built.

Some American cities, like Cincinnati and Rochester, were initially built around canals. When the railways replaced canals, they were able to bury a subway in the disused canal's trench, without rerouting other utilities, or acquiring a right of way piecemeal.

Another usual way is to start with a vertical shaft and then dig the tunnels horizontally from there, often with a tunnelling shield, thus avoiding almost any disturbance to existing streets, buildings, and utilities. But problems with ground water are more likely, and tunnelling through native bedrock may require blasting. (The first city to extensively use deep tunneling was London, where a thick sedimentary layer of clay largely avoids both problems.) The confined space in the tunnel also limits the machinery that can be used, but specialised tunnel-boring machines are now available to overcome this challenge. One disadvantage with this, however, is that the cost of tunnelling is much higher than building systems cut-and-cover, at-grade or elevated. Early tunnelling machines could not make tunnels large enough for conventional railway equipment, necessitating special low, round trains, such as are still used by most of the London Underground, which cannot install air conditioning on most of its lines because the amount of empty space between the trains and tunnel walls is so small.

The deepest metro system in the world was built in St. Petersburg, Russia. In this city, built in the marshland, stable soil starts more than 50 metres deep. Above that level the soil mostly consists of water-bearing finely dispersed sand. Because of this, only three stations out of nearly 60 are built near the ground level and three more above the ground. Some stations and tunnels lie as deep as 100–120 meters below the surface. However, the location of the world's deepest station is not as clear. Usually, the vertical distance between the ground level and the rail is used to represent the depth. Among the possible candidates are:

One advantage of deep tunnels is that they can dip in a basin-like profile between stations, without incurring significant extra costs due to having to dig deeper. This technique, also referred to as putting stations "on humps", allows gravity to assist the trains as they accelerate from one station and brake at the next. It was used as early as 1890 on parts of the City and South London Railway, and has been used many times since.

History

The earliest claimant to the title of "world's oldest subway tunnel" is on the Long Island Rail Road, which in 1850 bricked over an open cut to form the Atlantic Avenue Tunnel, carrying its line for about 2750 feet (850 m) under the streets of Brooklyn (now part of New York City). This tunnel was not a true subway, as it had no stations and was used for long-distance as well as suburban trains. Today the line would be categorized as regional rail.

The first real underground line in the sense discussed here was the Metropolitan Railway in London, which opened in 1863, using the era's most advanced propulsive technology: steam locomotives, specially designed to condense their exhaust steam when in the tunnels. It was an immediate success and many extensions followed; the Metropolitan eventually became an important part of the eventual London Underground system. Steam working underground lasted until 1905.

The first elevated railway in the world was the Ninth Avenue Elevated in New York City, opened in 1868 as a cable car and later converted for steam and then electric operation. Elevated railways were seen as a cheaper alternative to subways, but were often seen as dirty, ugly, and dangerous.

In 1870 short single-track lines opened in both New York and London using alternative technologies, neither of which was a success. In New York, Alfred Beach built a 95 m tunnel (with a single station and a dead end at the other end) to demonstrate pneumatic train operation; this operated until 1873, after which the tunnel became a rifle range and was then abandoned. The Beach subway was demolished in 1912 to build the BMT City Hall station. In London, the Tower Subway provided a crossing under the River Thames using a tiny cable car for the 410 m journey; the line closed in a matter of months and the tunnel was given over to pedestrians, later becoming a utility conduit.

The first deep-level underground line (other than the Tower Subway) was the City and South London Railway, which opened in 1890. Steam operation being considered ridiculous in view of the limited ventilation so far underground, cable traction was chosen; but during construction the management decided to try electric locomotives instead, and so the C&SLR became the first underground electric railway. It too is now part of the London Underground.

The first line of the Chicago 'L' opened in 1892; most of Chicago's system is still elevated, but a few subways have been built, the first opening in 1943.

The first underground railway in continental Europe was completed in Budapest in 1896, after only two years of construction, from Vörösmarty tér (the city centre) to Városliget and the local zoo. This line, now part of the Budapest Metro, stretched 3.7 km (2.3 mi). The 10.4 km (6.5 mi) Glasgow Subway in Scotland opened the same year and used cable haulage, until it was electrified in 1935.

The first line of the Paris Metro opened in 1900. Its full name was the Chemin de Fer Métropolitain, a direct translation into French of London's Metropolitan Railway. The name was shortened to métro, and many other languages have since borrowed this word.

Boston has the oldest subway tunnel in the United States, part of the Green Line downtown, dating from 1897. The original construction was a short four-track tunnel downtown, with only two stations, built to take light rail cars from outlying areas off the streets. Later subways in Boston carried full-size trains; the Green Line still operates with light rail equipment. In 1901, heavy rail trains began to use the tunnel as part of the original configuration of the Main Line Elevated, the first elevated railway in Boston.

The New York City Subway, which has become the world's largest (by some measures), did not open its first section until 1904, but this was a fully separate four-track line, stretching 9 miles (14.5 km)[1] from City Hall to 145th Street. Extensions were soon built, reaching the Bronx and Brooklyn; this is now part of the IRT system. Subway trains now run on right-of-way first used in 1863, and converted R44 cars run on the 1860Staten Island Railway.

Finance

Many early urban railways were originally constructed by private enterprise, either independently as in London, or under a government franchise as in Paris. Later, direct government control and ownership became the norm as corporations found it difficult to maintain profitability, although in recent years, this trend has been reversed to some extent with moves towards public-private partnership.

Whatever its original financing, any metro system requires ongoing expenditure to maintain and replace its trains, tracks, tunnels, and other infrastructure. An under-financed system will become unreliable and unpleasant to use, and eventually unsafe, and thus will lose public support.

In the United States, the lack of metro, subway, or other rapid transit systems except in a few of the larger cities has been attributed to the influence of the automotive industry and oil refining companies, which essentially used automobiles, buses, and advocacy of public road-funding to compete effectively against existing streetcar and trolley systems. These went into decline early in the twentieth century as the public chose personal over public transportation. (The decline of rapid transit outside of major metropolitan centers gave rise to the General Motors Streetcar Conspiracy, a conspiracy theory that auto and oil interests directly engineered the dismantling of rapid transit.)

Additionally, many streetcar/tram systems include underground and (less commonly) elevated sections, in which everything about the system except the right-of-way is built to streetcar standards. Notably, the first subway in the United States, Boston's Green Line, opened in 1897 to take streetcars off downtown streets, though it did carry elevated trains from 1901 until the Washington Street Subway opened.

The coming of modern light rail in the 1970s brought new crossovers. New systems were built and old streetcar/tram systems were upgraded with higher capacity and speeds, but retaining some aspects of streetcars and trams. Some systems known as light rail, such as the Docklands Light Railway in London and New York City's AirTrain JFK, are rapid transit with some light-rail technologies (though the light rail in these cases may be a mislabeling of advanced light rapid transit). Other light-rail systems may use high platforms but otherwise run as streetcars. A few systems similar to interurbans have even come back, such as New Jersey's River LINE, which operates over freight rails for most of its trip, and along streets on one end.